The naj was, according to Klingon spiritual beliefs, the "dream before dying".
According to Brok'tan, upon a Klingon's death, the mind, unprepared to face death, creates the illusion of life for the recently deceased. In 2376, he told B'Elanna Torres that her perception that she had just been on the USS Voyager was her naj. (VOY: "Barge of the Dead")